X-ray tubes are employed for example in X-ray systems for medical applications. An X-ray tube is used to generate electromagnetic radiation which may be used e.g. for medical imaging applications.
Regularly, electrons are accelerated between a cathode and an anode within an evacuated housing for producing X-rays. The electrons impinge on a part of the anode called the focal spot, thus creating electromagnetic radiation.
X-ray generation may be considered to be very inefficient, as a major part of the applied energy is converted to heat. The dissipation of heat, in particular at the focal spot, may be considered to be one of the central limitations of X-ray tubes.
By employing a rotating anode, the area of impingement of the electrons, the focal spot, may be considered to be a non-static area on the surface of the rotating anode disc.
Thus, by rotating the anode, the heat load acting on the focal spot and thus the anode may be spread over a larger area, increasing the power rating of the X-ray tube substantially.
An according rotating anode X-ray tube may generate X-radiation in a diagnostic system. The anode of the X-ray tube may heat up upon operation and may cool down afterwards. This thermal cycling may cause thermo-mechanical distortions of tube components so that the tubes may have to be designed to function reliably under all application conditions.
Thus, high-performance X-ray tubes may use hydrodynamic bearings to support the rotating anode while dissipating heat from the anode by direct conduction cooling towards an external cooling fluid. Due to the evacuated tube housing, other means for heat removal, e.g. by convection, may be difficult to achieve.
However, the thermal conductivity of an anode may be limited by a breathing “vacuum” gap between the anode disc and the bearing. An according gap may compensate expansion and/or reduction in size of the individual anode parts, in particular the disc-shaped anode element, due to the heating-up during operation and the cooling-down after operation of the X-ray system.
Furthermore, the “breathing” vacuum gaps may be required to align the anode and the bearing shaft to compensate for thermal stresses.